Transpiration

Transpiration is the term applied to the escape of water from leaves and other parts of the plant in the form of vapor.^CH

There is no closed "circulation" of sap in plants comparable to the streaming of blood in animals. Water which generally contains various mineral soil constituents in solution enters the roots, and most of it passes upward through the stems directly to the leaves, where it is evaporated. In plants of a succulent character, the intake often exceeds the loss, and a large amount of surplus liquid may be accumulated in various organs, from whence it may pass to the leaves in times of drought.^CH

The movement, of water set up by transpiration carries water and the contained salts to the centers of food formation in the leaves, and the evaporation process facilitates exchange of gases with the air. The evaporation also tends to equalize temperatures. That an enormous amount of work is performed by the plant in transpiration may be seen when it is known that a single sunflower plant will evaporate a pint of water from its leaves in a single day, and about seventy times this much in the course of its development. A birch tree with 200,000 leaves will transpire from 700 to 1,000 pounds of water daily in the summer. A single oak tree will throw 120 or 130 tons of water into the air during the course of a season, and an acre of beech trees containing 400 to 600 specimens will transpire about 2,000,000 pounds in a single summer. It is estimated that 98 per cent of the energy derived from sunlight by leaves is expended in the work of transpiration.^CH

To determine the exact amount of water transpired by a plant, a specimen not more than a yard in height, growing in a pot, may be used. Set the pot on a square of oilcloth, then bring the cloth up around the pot and tie closely to the stem of the plant. This will prevent evaporation except from the shoot. Now set the prepared plant on one pan of a scale, together with a small measuring-glass, and balance. Allow the plant to remain in the warm sunshine for eight hours, then note the amount of water which must be poured into the glass to reach the original level and restore the balance: this will represent the amount of transpiration.^CH

To demonstrate that water actually does come from the leaf, cut off a small leafy shoot of any convenient plant and thrust the base of the stem through a piece of cardboard into a tumbler of water: seal the opening around the stem with wax or gelatine: then cover the exposed part of the shoot with another tumbler and set in a warm light place. Moisture which could have come only from the leaves will soon gather on the glass. Some transpiration occurs over the entire surface of the plant, although only about one-thirtieth as much is given off by the stem as from the same amount of leaf-surface.^CH

The structure of the leaves is such as to facilitate transpiration. The interior of the leaf is made up of a great number of loosely arranged cells which evaporate water into the air between them. The air in the leaf communicates with the atmosphere through openings called stomata, which are generally placed on the lower side of the leaf. Consequently the watery vapor diffuses out through the stomatal opening. Near the outer end of the stomatal chimneys are guard-cells which undergo alterations in form that change the size of the opening of the guard-cells and modify the rate of water loss. The position and shape of the guard-cells are affected by the age of the leaf, turgidity of the tissues, wind, and sunlight. The rate of transpiration may also be affected by cutinization or other alterations in the cell-walls through which evaporation into the stomatal chambers takes place, or by the presence of colloids retentive of water in the cells.^CH

Species characteristic of arid regions generally have limited waterproofed surfaces with a comparatively small number of stomata. This modification is exemplified by the cacti, which transpire not more than one three-hundredth as much water as a broad-leaved plant of the same volume; and such succulents may lose as little as one forty-thousandth of their total weight by transpiration in a day.^CH